1981 rm125 issues

By kxjim, Posted April 23, 2016

Trying to get a 81 rm125 running to race in a vintage class we have going on. bike had new .030 piston installed 25 years ago, and was never started again.

I have spark, and fuel(maybe too much). removed carb and totally cleaned and blown out. removed cyl. and checked reeds and overall condition. Bottom end feels really good, problem is i cant get this thing to light whatsoever. the piston is a full-skirt design, and im wondering if the lack of skirt cutouts on the piston is affecting the ports in any way??

thanks in advance for all replies.

By 78SuzyQ, Posted April 23, 2016

so you say you have spark and fuel and you think you're getting too much fuel

shut off the fuel from the gas tank drain the carb bowl and spray some gasoline or carb cleaner into the carb intake and see if it starts

other than that check the compression

By kxjim, Posted April 23, 2016

compression seems low, although i didnt gauge test it. at rest, the reeds are not PERFECTLY closed. maybe couple thousandths open, although i would assume case pressure would close them down that much. With a new piston and rings and the correct size bore, what else can cause that loss of compression>?

By 78SuzyQ, Posted April 23, 2016

compression seems low, although i didnt gauge test it. at rest, the reeds are not PERFECTLY closed. maybe couple thousandths open, although i would assume case pressure would close them down that much. With a new piston and rings and the correct size bore, what else can cause that loss of compression>?

you said it had a .030 piston installed that looked new and that it was never started again (assuming that is true) there is a reason that it was never started again

without testing for compression with a compression tester how are you going to ever know ?

if the reeds aren't perfectly closed just take them out, put em back on flipped

when you took off the head did you compress the head nuts back on properly in a criss cross pattern ? are you using the right type of nut, this is the type of nut you should be using

a flange locknut with the grip pattern

are all the gaskets in order?

By 78SuzyQ, Posted April 23, 2016

I assume you checked the bore with a micrometer and checked the diameter of the cylinder

if i had to guess i'd guess the guy replaced the piston with the same size as before instead of boring out the cylinder up another size or the full skirt is blocking the intake port

Edited April 23, 2016 by 78SuzyQ

By kxjim, Posted April 24, 2016

cylinder bore and piston are correct size, i cant understand the port/reed in crankcase arrangement.. there are ports above the reeds that could hypothetically push the charge back into intake. Im mechanically inclined, and tend to figure stuff out eventually, but i cant grasp this without an explanation. reeds are not cracked or chipped, but seems charge is coming back thru the intake tract...

By kxjim, Posted April 24, 2016

figured it out. previous owner never tightened up head bolts. only fingered them snug... NOT TIGHTENED. went to pull head off, and found loose bolts... tightened them up and fired right off first kick... I feel kinda inferior now... but after cleaning carb 3 times and all the other stuff i did, it runs like a sewing machine. thanks for the help. JIM